The present invention relates to an apparatus for detecting obstructions of a flow line, in particular, the fluid flow line of a medical instrument.
Automated sample handling systems are known that automatically aspirate patient fluid samples, such as blood plasma, from sample tubes for subsequent monitoring or testing of the fluid sample. For example, in U.S. Pat. No. 5,236,666 to Hulette et al., entitled "Temperature Regulation in a Sample Handling System for an Optical Monitoring System," there is disclosed an automated sample handling system for an optical evaluation instrument that can handle a high throughput of patient samples with a high degree of versatility, adaptability, and reliability. Hulette et al. discloses a sample handling system which allows walk-away automation once sample tubes containing patient samples are loaded into the system. The sample tube is automatically advanced to a piercer where a piercing probe is caused to pierce the septum of the sample tube. A sample probe is lowered a predetermined distance into the tube to aspirate a programmed amount of sample. The sample probe is then removed from the sample tube and the sample subsequently dispensed into a cuvette.
Typically, the amount of fluid sample aspirated with automated sampling handling systems is relatively small, for example, 105 to 500 microliters. Precise aspiration of the sample from the sample tube is therefore critical. Due to the micro-amounts of fluid being aspirated, relatively small obstructions within the fluid sample, such as blood clots, can prevent the requisite amount of fluid from being drawn from the sample tube, resulting in inaccurate test results and decreasing the overall efficiency of the system.
Precision microfluid pumps, such as Cavro brand pump, manufactured by Cavro Scientific Instruments, Incorporated of Sunnyvale, Calif., have been developed that can accurately aspirate and dispense the aforementioned quantities. The Cavro brand pump is provided with a syringe having a plunger, and a stepper motor. To obtain the necessary precise volumetric delivery, the stepper motor moves the plunger a certain distance, for example, 0.0001 inch, aspirating an amount of fluid proportional to the distance moved.
When a pump begins a normal aspiration cycle, there is associated with this cycle within the fluid flow line an initial vacuum and a subsequent increase in pressure. When the pump, for example, the aforementioned Cavro brand pump, is turned on, the plunger within the pump is moved and begins drawing a vacuum. As fluid is drawn into the fluid flow line, the fluid continues to move until the plunger of the pump stops. Because a fluid in motion tends to stay in motion, when the moving fluid hits an immovable object, such as the piston, there is a resultant sudden pressure increase. However, if an obstruction, such as a blood clot, prevents the flow of the fluid within the flow line, this increase of pressure is absent.
In the past, an operator or technician would typically manually check the sample tube for obstructions by holding the sample tube up to the light and swishing the contents around while searching for foreign material. However, this method requires human intervention, diminishing the automation and flexibility of the aforementioned automated systems. Therefore, an automated device for detecting obstructions within a fluid flow line, minimizing human intervention, is desired.
The inventors experimented with an automated system of detecting obstructions in the fluid flow lines utilizing commercially available pressure detectors placed within the flow line of the medical instrument, in between the aspirating probe and the corresponding pump. If a pressure increase at the end of the aspiration cycle was not detected, this would indicate an obstruction of the fluid flow line, allowing the appropriate action to be taken. Stated alternatively, by monitoring the pressure signal associated with the fluid flow line, the presence or lack thereof of an obstruction could be determined.
It was discovered that the commercially available pressure detectors were either too delicate or lacked the sensitivity for this application. The instantaneous pressures that were developed due to the pumping action of the microfluid pumps often exceeded the burst pressure of the commercially available pressure sensors, causing them to break or malfunction.